Document FR3010951 discloses a rack of molded plastic, comprising a series of notches open in a first direction, each notch having a certain width in a second direction substantially perpendicular to the first direction, said notches being aligned in a third direction substantially perpendicular to the first and second directions, and each notch being defined by a bottom and by first and second side walls extending in the first direction from the bottom to a mouth of the notch.
Such racks are used in particular for adjusting the height of headrests, where a portion of the headrest slides along the rack and can be locked in position by engaging a locking member (wire or other) in one of the notches of the rack.
Demolding constraints sometimes require creating the notches with some flaring towards their mouth, such that the metal wire can sometimes jump from one notch to another in the event of an accident or when the wire is placed in the locking position while the headrest is being moved vertically. This effect may possibly be permitted or even desired in one direction, for example when moving the headrest upwards; however, it is not permitted in the opposite direction, for example when moving the headrest downwards, as this can be dangerous for the user or at least results in inaccuracy in the height adjustment of the headrest.